1. Field of Invention
The present invention relates to optical systems and, more particularly, is directed towards uni-directional viewing systems.
2. Description of the Prior Art
In a wide variety of security and crime prevention applications, it is desirable to provide for the observation of persons in such a manner that the individual being observed cannot detect the presence of the observer. Primarily, such applications are: (1) provisions for observation in business establishments and other institutions for the detection and prevention of thefts, holdups, burglaries, and other crimes, and (2) provisions for the observation of residential doorway areas for the indentification of persons wishing to gain entrance and for the prevention of crime. Other applications include guard houses, prisons, armored cars, and equipment for human and animal behavioral experiments.
In business establishments and other institutions, a number of techniques and devices are used for the observation and prevention of thefts and other crimes by shoplifters, customers, employees, delivery personnel, and others. Among the important methods utilized to reduce crime are (1) employment of uniformed and plainclothesmen, (2) use of both visible and unobtrusive oneway viewing devices such as transparent mirrors and closed-circuit television systems, (3) use of small detachable and electronically detectable devices attached to unsold merchandise, and (4) building and urban design.
A widely employed viewing device for the detection of crime in business establishments and other institutions involves the use of semi-reflective, semi-transparent plate glass sheets also known as transparent mirrors, beam splitter mirrors, two-way mirrors, or one-way mirrors. Such devices partially transmit and partially reflect light. If the area on one side of the glass is in an illuminated environment and the area on the other side of the glass is in a substantially darker environment, the area on the light side is visable through the glass and the area on the dark side is not visable through the glass, the illuminated side being operative as a mirror. Thus, an important disadvantage of such a device is that it requires the observer's area to be kept much darker than the area to be observed. This generally requires the use of a large amount of space for darkened rooms and makes casual observation by a person engaged primarily in other tasks not practical.
In the residential security field, small observation scopes, or peepholes, are mounted in apartment or house doors to provide for the identification of those wishing to gain entrance. The major disadvantages of such devices include: (1) a limited viewing angle, (2) optical distortion of the scene to be viewed if a wide angle lens is used to increase the viewing angle, (3) the ability of the individual being viewed to detect that he is being observed and (4) a limited viewing surface so that the person being observed usually must make his presence known in some other way and casual observation for the prevention of crime in hallways and doorway areas is made impracticable.
Each of these one-way viewing devices, with the exception of small observation scopes, provides the advantage that the person being viewed is unaware that he is under surveillance at any given time. Thus, the occasional use or even the mere presence of such devices may be sufficient to prevent crimes. At times, dummy television cameras are utilized with this purpose in mind.
In some applications, such as retail stores handling expensive merchandise, it is important to provide for unobtrusive viewing in the sense that the individuals do not know that they are under surveillance. In such cases, transparent mirrors have been disguised as conventional mirrors, decorative fixtures, or other objects. Sometimes, scanning television cameras are mounted within such decorative fixtures.
Closed circuit television systems for both institutional and residential use provide unobtrusive and casual observation with a large viewing area. Furthermore, systems employing large numbers of cameras and monitor screens covering different viewing angles and areas can be constructed. However, such systems are relatively expensive and unduly complex. Optical resolution is imperfect and provision for color viewing and depth perception is costly. Generally, cameras are too large to be mounted within doors or other small spaces.